Modeling of Fine Root Biomass of Two Dominant Tree Species in the Mixed Broad-Leaved Korean Pine Forest

Quantifying fine root biomass is essential for comprehending forest subsurface ecosystems and predicting changes in plant biomass allocation under heterogeneous soil nutrient conditions. However, little is known about fine root biomass, especially the quantitative analysis of species-specific fine root biomass, due to the destructive, time-consuming, and costly nature of the associated sampling methods. In this study, nested regression was used to estimate the biomass and surface area of fine roots of individual trees of the two main tree species, Pinus koraiensis and Fraxinus mandschurica, in the primary broad-leaved Korean pine forest. Additionally, the relationship between aboveground traits (leaf biomass and leaf area) and belowground traits (root biomass and surface area of fine roots) was further analyzed. The results show that the fine and total roots of both species were highly fitted to the diameter at breast height with R2 = 0.92–0.97. The root-to-shoot ratios at the single-plant level were all 0.20. Significant linear relationships existed between leaf biomass and surface area and between fine root biomass and surface area (adjusted R2 = 0.95, p < 0.001). In conclusion, the tight coupling between fine roots and leaves proposed in this study, together with the developed biomass equation, can be used to accurately estimate Pinus koraiensis and Fraxinus mandschurica biomass at the stand level in primary broad-leaved Korean pine forests.